This is not quite true with the right tactics players sometimes have 2 opponents on them
i.e. your SG has the ball and has the opposing PG and SG covering them and the SF is covering your PG
I think it has to do with performance within the game as well. If you SG is playing really badly then he will be less likely to have 2 players guarding him.
If both teams play Man to Man and Base Offense then the battle will be individual against individual most of the time.